Residual strength and stiffness of compact bovine bone specimens during fatigue were investigated and the fracture surfaces were examined by scanning electron microscopy. Residual strength tests consisted of rotating bending fatigue followed by tensile tests to failure. Specimens loaded to 50% of their fatigue lives had approximately a 13% loss of tensile strength (p < 0.01). Specimens tested in zero to tension axial fatigue showed a gradual loss of stiffness which in some specimens was greater than 20% at failure. These results indicate that repeated loading of bone causes a progressive loss of strength and stiffness. A similar fatigue behavior in composite materials is attributable to cumulative microcracking, debonding, void growth, and fiber breakage.